High modulability for ac and dc components is required for numerous applications of magnet cores in which, depending on the case, specific modulability for ac and dc is necessary. Applications of magnet cores with high modulability for ac and dc components are present, for example, in current transformers and current-compensated inductors.
Current-compensated noise suppression inductors are described in DE-A 35 26 047 and DE 195 48 530 A1. They have two windings for one-phase application and three or more windings for multiphase applications. The windings of noise suppression inductors are connected so that the magnetic fluxes that are induced by the operating current rise mutually, whereas interference currents that flow with the same phase through the two windings result in magnetization of the soft magnetic core. Because of this the current-compensated noise suppression inductor produced acts as a very small inductive resistance with reference to the operating currents, whereas interference currents, which come from connected equipment, for example, and are closed via the ground, encounter very high inductance.
The core of the known current-compensated noise suppression inductors is produced from amorphous or amount of crystalline alloys, preferably band material. The inductance of the inductor then depends essentially on the relative permeability of the soft magnetic material of the magnet core, in addition to the number of windings and the core cross section.
Current transformers with the magnet cores mentioned in the introduction can be used in watt meters, as described for example in WO 00/30131. Watt meters are used, for example, to record power consumption of electrical equipment and installations in industry and the household. The oldest useful principle is the Ferraris meter. The Ferraris meter is based on power metering via the rotation of a disk connected to a mechanical meter, which is driven by the current- or voltage-proportional fields of corresponding field coils. For expansion of the functional capabilities of watt meters, like multiple rate operation or remote reading, electronic watt meters are used in which current and voltage recording occurs via current and voltage converters. The output signals of these converters are digitized, multiplied, integrated and stored; the result is an electrical quantity that is available for remote reading.
One of the possible technical variants of such a current converter is the current transformer according to the induction principle. FIG. 1 shows a substitution circuit where this type of current transformer and the ranges of the technical data can occur in different applications. A current transformer 1 is shown here. The primary winding 2, which carries the current Iprim to be measured and a secondary winding 3, which carries the secondary current Isec are situated on a magnet core constructed from a soft magnetic material. This current Isec is automatically adjusted so that the primary and secondary ampere turns in the ideal case are the same size and oppositely directed. The trend of the magnetic fields in such a current transformer shown in FIG. 2, in which the losses in the magnet core are not considered because of their generally low value. The current in the secondary winding 3 is then set according to the law of induction so that it attempts to prevent the cause of its formation, namely the time change of the magnetic flux in magnet core 4.
In the ideal current transformer the secondary current, multiplied by the ratio of number of windings is therefore negatively equal to the primary current, which is explained by equation (1):
                              I          sec          ideal                =                  -                                    I              prim                        ⁡                          (                                                N                  prim                                                  N                  sec                                            )                                                          (        1        )            
This ideal case is never reached because of losses in the load resistance 5, in copper resistance 6 of the secondary winding and in the magnet core 4.
In real current transformers the secondary current therefore has an amplitude error and a phase error relative to the above idealization, which is described by equation (2):
                                          Amplitude            ⁢                                                  ⁢                          error              :                              F                ⁡                                  (                  I                  )                                                              =                                                    I                sec                real                            -                              I                sec                ideal                                                    I              sec              ideal                                      ;                              Phase            ⁢                                                  ⁢                          error              :                              φ                ⁡                                  (                  I                  )                                                              =                                    φ              ⁡                              (                                  I                  sec                  real                                )                                      -                          φ              ⁡                              (                                  -                                      I                    prim                                                  )                                                                        (        2        )            
The output signals of such a current transformer are digitized and further processed in the electronics of the watt meter.
The electronic watt meters used for power metering in industrial applications operate indirectly because of the often very high (>>100 A), i.e., special primary current transformers are connected in front of the current input so that only pure bipolar, zero-symmetric alternating currents (typically 1 . . . 6 Aeff) need be measured in the counter itself. For this purpose current transformers are used constructed from magnet cores of highly-permeable materials, for example nickel-iron alloys containing about 80 wt % nickel and known under the name “Permalloy”. These have in principle a very low phase error φ to achieve low measurement errors, for which reason they are also equipped with very many (typically more than 1000) secondary windings.
For use in household meters, which can also be used in small industrial installations, these are not suitable, since with the usual direct connection without primary current transformers connected in front to current intensities can generally be 100 A and more and because of this the above described current transformers will be saturated. In addition, these currents can contain non-zero-symmetric dc fractions that are generated by semiconductor circuits used in modern electrical equipment (for example, rectifier or phase control circuits) and which saturate current transformers with highly permeable magnet cores magnetically and therefore distort the power metering.
The international standards that apply for this of the IEC 62053 stipulate that an electronic watt meter must be able to measure a maximum amplitude of a unipolar half-wave rectified sinusoidal current with a maximum additional error of 3 or 6% to comply with the accuracy classes 1 and 2% for a stipulated maximum measurable effective value Imax of a bipolar zero-symmetric sinusoidal current, the numerical value of which is equal to the maximum effective value. In addition to these standards there are regional and national provisions that permit as sufficiently precisely defined behavior power recording even with a low amplitude limit value of the unipolar current.
To form such current, current converters are known which operates on the basis of open magnetic circuits or low-permeability magnetic circuits sheared with mechanically introduced air gaps. An example of such a current converter is a current transformer in which a ferrite-shell core provided with an air gap (sheared) is used as magnet core. This has satisfactory linearity as a function of primary current, but because of the relatively low saturation induction of ferrite a comparatively large volume magnetic core is required in order to achieve a high maximum measurable primary current with high linearity over the entire current range in the current transformers. These current transformers also have high sensitivity to external foreign fields so that fielding measures must also be taken, which are material- and installation-intensive and therefore not very favorable in terms of cost. In addition, the magnetic values are generally strongly temperature dependent, in ferrites.
Current converters are also known that operate on the basis of iron-free air coils. This principle is known as the so-called Rogowski principle. The effect of the properties of a soft magnetic material on measurement accuracy drops out here. Owing to the magnetically open design of such current converters, they must be equipped with particularly demanding shields against external fields, which is also cost-intensive because of the material and installation expenditure.
A technically high-value possibility for implementation is the use of current transformers with relatively low permeability (μ=1400-3000) magnet cores from fast-solidified amorphous soft magnetic materials. The very good constancy of this permeability during changes in the level control guarantees very high linearity of the phase error over the entire current range to be transmitted. Because of the low permeability value saturation with the dc fractions is avoided within calculable limits; on the other hand, it leads to the occurrence of a comparatively high phase error between the primary and secondary current, which must be compensated in watt meters by a corresponding electronic circuit or software. In previously known variants of electronic watt meters a compensation range of typically 0.5-5° is present, in which compensation of higher values of this range, however, requires increasing demands with reference to signal processing semiconductor circuits and memories, which increases the equipment cost. A serious problem from the standpoint of manufacturers competing on the market for watt meters are the costs for the magnetic materials to be used, since the previously used alloys contain about 80 atom % Co, which leads to a comparatively high material price.